If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
Synonyms: de novo fatty acid biosynthesis, initial reactions
|Superclasses:||Biosynthesis → Fatty Acid and Lipid Biosynthesis → Fatty Acid Biosynthesis|
Fatty acids are key building blocks for the phospholipid components of cell membranes and are determinants of intracellular communication, in the form of lipid second messengers [Prieschl00], and fatty acyl moieties of proteins that modify their location and function [Resh99].
There are two basic types of fatty acid (FAS) biosynthesis mechanisms. The type I system is found in mammals and lower eukaryotes. Type II systems are found in bacteria, plants [White05], parasites of the Apicomplexa phylum [Ferguson07a] and mitochondria [Zhang03h, Miinalainen03]. The reactions in these systems are catalyzed by a series of individual soluble proteins that are each encoded by a discrete gene, and the pathway intermediates are transferred between the enzymes as thioesters of a holo-[acp].
The best-studied pathway is that of E. coli K-12, from which all the enzymes have been purified and crystallized [White05].
Fatty acid biosynthesis starts with an initiation sequence that produces an acetoacetyl-[acp], an activated molecule that is used for subsequent elongation reactions, which ultimately produce the final fatty acid products. This pathway describes only the initiation sequnce, subsequent elongation reactions are described in the pathways fatty acid elongation -- saturated and (5Z)-dodec-5-enoate biosynthesis.
About This Pathway
E. coli has several different routes in which it can produce acetoacetyl-ACP, and this pathway describes the major route. In it, one acetyl-CoA molecule is converted to malonyl-CoA, which is then activated to malonyl-ACP that is condensed with a second acetyl-CoA molecule to form acetoacetyl-ACP.
The first enzyme in this pathway, acetyl-CoA carboxylase, is the controlling point for the flux of carbon into lipids. It is a biotin-containing multi-enzyme complex with two activities, biotin carboxylase and acetyl-CoA carboxyltransferase, catalyzing two half reactions that combined condense bicarbonate with acetyl-CoA to yield malonyl-CoA. Further discussion of this enzyme is available in the pathway biotin-carboxyl carrier protein assembly.
The malonyl-CoA is next attached to a holo-[acp] by the enzyme malonyl-CoA-ACP transacylase.
The last step of the pathway, the condensation of a second acetyl-CoA with malonyl-ACP, is carried out by the condensing enzyme β-ketoacyl-ACP synthase (KAS). E. coli has three β-ketoacyl-ACP synthases: KASI, KASII and KASIII encoded by fabB, fabF and fabH respectively.
KASIII is the key enzyme in the initiation of fatty acids biosynthesis. It selectively catalyzes the formation of acetoacetyl-ACP and specifically uses CoA thioesters rather than acyl-ACP as the primer. The products tend to be shorter than the products of KASI and II, which are involved primarily in elongation reactions. Unlike the other two enzymes, KASIII cannot participate in the terminal elongation steps of fatty acid biosynthesis [White05].
KASIII is inhibited by long-chain acyl-ACPs, indicating a role in feedback regulation of fatty acid synthesis. Since it catalyzes the first condensation step in fatty acid synthesis, it is able to control the rate of fatty acid initiation.
The substrate specificity of KASIII is a major determining factor in membrane fatty acid composition of an organism. In E. coli the enzyme is highly selective for acetyl-CoA, and thus only straight-chain fatty acids are produced in this organism. [Choi00].
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Ferguson07a: Ferguson DJ, Campbell SA, Henriquez FL, Phan L, Mui E, Richards TA, Muench SP, Allary M, Lu JZ, Prigge ST, Tomley F, Shirley MW, Rice DW, McLeod R, Roberts CW (2007). "Enzymes of type II fatty acid synthesis and apicoplast differentiation and division in Eimeria tenella." Int J Parasitol 37(1);33-51. PMID: 17112527
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Zhang03h: Zhang L, Joshi AK, Smith S (2003). "Cloning, expression, characterization, and interaction of two components of a human mitochondrial fatty acid synthase. Malonyltransferase and acyl carrier protein." J Biol Chem 278(41);40067-74. PMID: 12882974
AbdelHamid07: Abdel-Hamid AM, Cronan JE (2007). "Coordinate expression of the acetyl coenzyme A carboxylase genes, accB and accC, is necessary for normal regulation of biotin synthesis in Escherichia coli." J Bacteriol 189(2);369-76. PMID: 17056747
Alhamadsheh07: Alhamadsheh MM, Musayev F, Komissarov AA, Sachdeva S, Wright HT, Scarsdale N, Florova G, Reynolds KA (2007). "Alkyl-CoA disulfides as inhibitors and mechanistic probes for FabH enzymes." Chem Biol 14(5);513-24. PMID: 17524982
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Blanchard99a: Blanchard CZ, Chapman-Smith A, Wallace JC, Waldrop GL (1999). "The biotin domain peptide from the biotin carboxyl carrier protein of Escherichia coli acetyl-CoA carboxylase causes a marked increase in the catalytic efficiency of biotin carboxylase and carboxyltransferase relative to free biotin." J Biol Chem 1999;274(45);31767-9. PMID: 10542197
Bognar87: Bognar AL, Osborne C, Shane B (1987). "Primary structure of the Escherichia coli folC gene and its folylpolyglutamate synthetase-dihydrofolate synthetase product and regulation of expression by an upstream gene." J Biol Chem 262(25);12337-43. PMID: 3040739
Butland05: Butland G, Peregrin-Alvarez JM, Li J, Yang W, Yang X, Canadien V, Starostine A, Richards D, Beattie B, Krogan N, Davey M, Parkinson J, Greenblatt J, Emili A (2005). "Interaction network containing conserved and essential protein complexes in Escherichia coli." Nature 433(7025);531-7. PMID: 15690043
Daines03: Daines RA, Pendrak I, Sham K, Van Aller GS, Konstantinidis AK, Lonsdale JT, Janson CA, Qiu X, Brandt M, Khandekar SS, Silverman C, Head MS (2003). "First X-ray cocrystal structure of a bacterial FabH condensing enzyme and a small molecule inhibitor achieved using rational design and homology modeling." J Med Chem 46(1);5-8. PMID: 12502353
Davies00: Davies C, Heath RJ, White SW, Rock CO (2000). "The 1.8 A crystal structure and active-site architecture of beta-ketoacyl-acyl carrier protein synthase III (FabH) from escherichia coli." Structure 8(2);185-95. PMID: 10673437
Davis00: Davis MS, Solbiati J, Cronan JE (2000). "Overproduction of acetyl-CoA carboxylase activity increases the rate of fatty acid biosynthesis in Escherichia coli." J Biol Chem 275(37);28593-8. PMID: 10893421
DiazMejia09: Diaz-Mejia JJ, Babu M, Emili A (2009). "Computational and experimental approaches to chart the Escherichia coli cell-envelope-associated proteome and interactome." FEMS Microbiol Rev 33(1);66-97. PMID: 19054114
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